First, meet the cliff swallow. These sweet little birds use mud to build bubble-shaped nests that hang from walls and ceilings. They also eat bugs that they catch flying in mid-air. The little bugs try their best to dodge being eaten, but the birds are able to turn and dive in tight patterns to catch them. Cliff swallows' wings are triangular, short, and pointy, which are good for close maneuvering like that.
Anti-gravity housekeeping at its very cutest.
But here's the thing. Cliff swallows love to nest near highways. This brings them close to lots of speeding cars, and
-- BAM! --
-- BAM! --
plenty of cliff swallows become roadkill. (Brief pause here to be sad about that. Poor birdies!)
So over thousands of years, these birds evolved wings to be able to maneuver in the air and catch bugs. But then highways were built, and suddenly huge hunks of metal were zooming through the neighborhood way faster than bugs. The birds had a new way to die (BAM!) or if you think of it another way, a new way to survive (dodge that huge speeding metal thingy!)
Birds with shorter wings can dodge and turn more quickly. Birds who can turn more quickly can better dodge the cars. The birds who can better dodge the cars have a better chance of surviving. Hmmm, it's starting to look like some evolution might happen...
Meanwhile, meet Charles and Mary Brown. They study cliff swallows at a research station in Nebraska that is chock full of the birds. Charles and Mary always pick up roadkill. Here's what they say in one paper:
"As we traveled daily in the course of our research, we stopped for each road-killed cliff swallow. We made the same effort to search for road kills and drove the same roads each year."
How many years did they do this, you ask? Thirty.
|Charles and Mary. Ah, biology love.|
Yes, you read that right. Thirty years. Think about that. A woman and her husband, driving the same roads for thirty years, picking up roadkill. For science. Was it romantic? Did the car smell bad? I don't know, but I love these people.
Anyway, while they picked up the dead swallows, they were also studying the living ones. So now they have thirty years of information on both the birds that got killed by cars and the birds that didn't. And they noticed that fewer and fewer birds were getting killed by cars. Could it be that the birds were evolving the ability to dodge the cars?
Yup. After thirty years and many generations, the cliff swallows' wings got shorter on average. As the longer-winged birds got killed off more often, the shorter-winged ones were left to produce the next generation. And on it went, so that fewer and fewer birds had longer wings, so fewer and fewer were killed.
This is a perfect example of how natural selection works:
- There's a change in the environment. (That's the cars zooming through like they own the place.)
- Some animals have an advantage in surviving in the new environment. (Shorter wings.)
- The animals who don't have the advantage (longer wings) die more often.
- The animals that survive longer have more kids than the ones that die. (Well, duh.)
- The animals born to the survivors have that same advantage. (More baby birds have shorter wings like their parents.)
- After several generations, more of the animals have that advantage. (Thirty years later, everybody's wings are shorter.)
So there you have it, kids. Proof that when two biologists fall in love, they will do nerdy and excellent things together for a long time.
Oh, and also that evolution is happening around us all the time.